Thermoplastic processable starch or starch derivative polymer mixtures

ABSTRACT

The present invention relates to thermoplastically processible esterification or transesterification products of starch or of starch derivatives with, for example, lactones, fatty acids, esteramides and the like, wherein the starch or the starch derivative is brought to melt using appropriate softeners or plasticizers before esterification or transesterification.

Applicant claims the benefit under 35 U.S.C. § 120 of earlier filedInternational Patent Application No. PCT/IB97/00915, filed Jul. 23,1997, pursuant to 35 U.S.C. §§ 363 and 371. Applicant also claims thebenefit under 35 U.S.C. § 119 of earlier filed Swiss Application No.1965/96, filed Aug. 9, 1996. For purposes of disclosure, the foregoingapplications are incorporated herein by specific reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a process for preparingthermoplastically processible starch polymer mixtures or starchderivative mixtures according to the preamble of claim 1 and to a numberof uses.

The present invention in particular relates to thermoplasticallyprocessible transesterification products of starch or derivativesthereof with, for example, lactones, esteramides, fatty acids, etc.and/or polyesters or other biologically degradable hydrophobic polymersand mixtures of transesterification products with the above-mentionedpolymers. Their use is based on the fact that numerous types of starch,as macromolecular raw materials, are cheaper than the knownthermoplastic materials.

Additionally, the present invention relates to processes for preparingthe transesterification products of starch or derivatives, such as, inparticular, starch acetates.

2. The Relevant Technology

Thermoplastically processible blends of starch have been described in WO90/05161. Thermoplastically processible blends of starch, of softenersand of water-insoluble polymers display only limited uptake of water andhave useful mechanical strength, but they are not storable at any levelof surrounding humidity in the long term.

In addition, in JP 05 125 101, transesterification products of starchhave been described which melt in the temperature range of from 150 to170° C.; however, these products are unsuitable for wide use owing totheir water uptake from the surrounding air and their poor mechanicalproperties. The preparation of these products by the known processes isprohibitive for wide use.

BRIEF SUMMARY OF THE INVENTION

The present invention, accordingly, proposes mixtures oftransesterification products of starch or derivatives thereof withlow-molecular-weight lactones, esteramides, fatty acids, etc., andoligomeric esters, polyesters and other hydrophobic biologicallydegradable polymers. At the phase boundaries between thetransesterification products and, for example, the polyester, thesemixtures show no preferred cracking on deformation and good stability ofthe mechanical properties on storage, and additionally they do notrelease any low-molecular-weight substances on contact with moisture.

The present invention additionally relates to a process for preparingtransesterification products of starch or derivatives, such as, inparticular, starch acetates with, for example, lactones and/orpolyesters in the presence of transesterification catalysts.

Transesterification products of starch with low-molecular-weightlactones, such as dilactide, caprolactone (CL) or diglycolide are known.Also known are transesterification products of starch and polyesters,such as polycaprolactone (PCL). The polyester component may additionallybe a copolyester, for example constructed from terephthalic acid, adipicacid, ethylene glycol and butanediol or from oligomeric esters ofethylene glycol and terephthalate, transesterified with caprolactone.However, the polyesters used have to be meltable in a temperature rangeof from 60 to 200° C.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention describes transesterification products of starchor of starch derivatives which can be processed with suitable polyestersand other biologically degradable hydrophobic polymers to givethermoplastically processible mixtures having useful properties. Suchmixtures consist of a disperse phase of the esterification ortransesterification product of the starch and a continuous phase of thepolyester or the hydrophobic, biologically degradable polymer. To beable to achieve suitable mechanical properties and storage stability ofthe mixture at various levels of surrounding humidity, macromolecularcomponents are required whose molecules bridge the phase boundariesbetween the disperse and the continuous phase. If such so-calledcompatibilizers are not incorporated into the mixture in sufficientamounts, thermoplasts having set breaking points at the phase boundariesare formed. It is a feature of the present invention that thecompatibilizers are advantageously formed at the phase boundaries whichare already present, that is to say that, if the compatibilizers areadded to the mixture of starch ester and polyester or hydrophobicpolymer, greater proportions of compatibilizers are required thanotherwise. It is furthermore crucial that the polyester or copolymersubstituents of the starch have the same chemical composition as thepolyester or polymer component of the mixture or a chemical compositionwhich is similar thereto. This ensures miscibility of the polyesters orpolymers. Furthermore, it has been found that, for example, thetransesterification products of starch with low-molecular-weightlactones and polyesters have considerably lower strength than thepolyphase mixtures just described. This is the case even when bothincorporate the same proportions of macromolecular polyesters; in thesecond case in the form of free macromolecules and a much lowerproportion in the form of block copolymers, in the first caseexclusively in the form of block copolymers.

Thus, according to a first aspect of the present invention, mixtures ofpolyesters and transesterification products of starch or of starchderivatives are proposed. On average, the molecules of thetransesterification products are constructed as follows: from 0.4 to 0.6parts by weight of starch radicals, from 0.6 to 0.4 parts by weight oflow-molecular-weight ester radicals and from 0.01 to 0.05 parts byweight of high-molecular-weight ester radicals.

In the present invention, particular attention was paid to thepreparation process. The starch is transesterified using partially orcompletely molten starch or molten starch derivatives, such as, inparticular, starch acetates. Hardly any of the known solvents orswelling agents for the starch which are required as additives formelting the starch can be removed by simple methods such asvolatilization from the reaction mixture after the reaction has ended.Water or formic acid, for example, are an exception here. However, incompetition with the starch, water also reacts with the transesterifyingagents. The water is therefore only allowed to remain in the systemuntil a certain low degree of transesterification of the starch has beenreached, and it is subsequently removed by volatilization. At thispoint, the starch is already present as transesterification product, andit is partially or completely molten at the temperature of the reactionmixture after the removal of the water. The transesterification of thestarch reaches a degree of substitution of from 0.8 to 1, i.e. onaverage from 0.8 to 1 of the three hydroxyl groups of the anhydroglucoseunit in the starch molecule are esterified.

An essential feature of the process according to the invention is themixing of the reaction mixture to shorten the required diffusion pathsof the molecular reaction partners. Furthermore the starch or thederivative, unless directly meltable, should be melted with the leastpossible amount of water, for example. These processes and thevolatilization of the reaction material at the appropriate time canadvantageously be carried out in a continuously operated kneader, forexample a twin-screw kneader having corotating screws. The screwelements are advantageously exchangeable; the melting process of thestarch requires kneader elements, the volatilization requirescompression and decompression elements and the reduction of the lengthof the molecular diffusion paths requires mixing elements.

According to a further embodiment, it is proposed to bring the starch orderivatives thereof into the melt using formic acid, and to react themsubsequently with lactones, such as, in particular, caprolactone. Here,the amount of water of the starch can be varied from virtually drystarch to about 25% by weight. However, the water content of theoriginal starch is preferably at most 10%, preferably 2-5%. Theproperties of the starch polymer mixture, such as, in particular, thestarch formate hydroxycaproate formed can be influenced by theproportion of water. Thus, the higher the water content, the moreformate is formed, and correspondingly, the lower the water content, themore caproate is formed. The thermoplastic processibility is the betterthe more caprolactone is reacted. In contrast, if more formic acid isreacted, the resistance to water is better. Here also, it is true thatthe water or the formic acid has to be stripped from the reactionmixture at least partially after a certain degree of substitution of thebasic molar unit of the starch has been reached.

Using this last-mentioned course of the reaction, it is possible toreplace a certain proportion of caprolactone, which is still relativelyexpensive; however, this is only possible to the extent to which areduction of the thermoplastic processibility is acceptable.

Again according to a further embodiment of the invention, it is proposedto employ, instead of starch, a starch acetate which is meltable inprinciple without adding an additional softener or plasticizer.Respectively, it is possible to incipiently swell a starch acetatedirectly by means of the transesterification reaction partner or tobring it into a meltable state, as is possible, for example, by adding alactone, such as, for example, caprolactone. By using starch acetates,it is possible to carry out the reaction homogeneously, i.e. withoutusing an additional solvent, such as, for example, water or formic acid,as is imperative if pure starch is employed.

A further advantage of using starch acetates consists in the fact thatthis class of compounds, such as, in particular, starch diacetate, isreadily commercially available, and at reasonable prices. Thus, forexample, starch diacetates which have a degree of substitution in anorder of magnitude of 1.9-2.3 and which on their own can hardly bemelted, but which can be melted together with caprolactone in atemperature range <200° C., thus making transesterification possible,can be obtained easily. In contrast, pure starch together withcaprolactone cannot be melted, owing to which in the latter caseaddition of a solvent or softener is always required.

Esterification or transesterification products of starch or of starchderivatives proposed according to the invention are suitable for mixingwith a number of other polymers, and the presence of theseesterification or transesterification products ensures the miscibilityof the starch or of starch derivatives with additional polymers. Asmentioned at the outset, these esterification or transesterificationproducts serve in this case as compatibilizers. However, theseesterification or transesterification products can also be used on theirown as thermoplastically processible polymers.

Here, starch or the esterification or transesterification product ispreferably mixed with hydrophobic biologically degradable polymers, suchas, for example, polyesters, copolyesters with aliphatic and aromaticblocks, polyesteramides, polyesterurethanes, polyvinyl alcohol,ethylenevinyl alcohol and/or mixtures thereof. Particularly suitable arepolycaprolactone, polylactides, polyhydroxybutyric acid and alsocopolymers with valeric acid and/or polyesters prepared by fermentation.

Other biologically degradable polymers suitable for use as mixturecomponents are natural polymers, such as gelatin, lignin, cellulose,derivatives of the above-mentioned materials and/or mixtures thereof.

Furthermore, it is possible to add fillers, fibers and other additivesto these polymer mixtures, as is generally customary in the plastics- orpolymer-processing industry.

The invention is now illustrated using the enclosed examples, but theseexamples are only intended to explain the present invention moreclearly, and not to limit the present invention.

EXAMPLE 1

5 kg of starch, 1.5 kg of water, 5 kg of caprolactone (CL) and 0.25 kgof 1,8-diazabicyclo(5.4.0)undec-7-ene (DBU) are melted and extruded in atwin-screw extruder with L/D=20 and D=46 mm, at 110° C. and a rotationalspeed of the screws of 50/minutes. The average dwelling time of thematerial in the extruder was 2.5 minutes. The extrusion process wasrepeated several times using the same material, in each case at theextruder temperatures: 110, 120, 140 and 160° C. After these transits,the material was volatilized at 180° C. in the next transit and fivetransits were subsequently carried out at 180° C. The extrudate wasextracted 4 times using dioxane. The dioxane solution contained theunreacted CL, the PCL and the DBU. The ratio by mass of the CL which hadbeen esterified with starch to the CL originally employed was 0.25 afterthe 5th transit and 0.87 after the 11th transit. The dioxane solutioncontained no PCL. The proportion of CL in the purifiedtransesterification product was determined using ¹ H NMR spectroscopy ind₆ -dimethyl sulfoxide solution at 80° C. and IR spectroscopy and was0.2 parts by mass after the 5th transit and 0.47 parts by mass after the11th transit.

The transesterification product having a proportion of 0.47 parts bymass of CL was reacted as above, but in only 3 transits at 180° C., withPCL in a mass ratio of 1:1. The proportion of PCL in the newtransesterification product was 0.02 parts by mass.

This product contained 0.48 parts by mass of unreacted PCL and wasexamined for strength in a simple tensile test at 20° C. and a take offspeed of 10 cm/minute. In all tests, the tension under pressure was >30MPA. The water uptake at 20° C. and a water activity of 1 was 0.03 partsby mass of water after 20 hours.

EXAMPLE 2

In a further experiment, the method of Example 1 was used, butvolatilization was carried out after two transits at 160° C., and theprocedure of experiment 1 was then followed. Within the margin of errorof the methods, the analytical results and the properties of the endproduct were identical.

EXAMPLE 3

Experimental

Native potato starch (25 g) (H₂ O content between 2 and 25%) was meltedwith formic acid (10 g) at 120° C. in a chamber kneader at 30 rpm. At awater content of 9%, 2 tests were carried out to investigate thereproducibility of the results. After 5 min, caprolactone (25 g) wasadded to the homogeneous clear melt. Samples were taken after 60 and 120min and extracted with hot dioxane (3 times) to remove the unreactedacylating agent.

Results

The proportion of caprolactone or formide was determined at roomtemperature and at 80° C. using ¹ H NMR. Owing to the ambiguous heightof the integrals of the starch signals, a maximum value and a minimumvalue for the detected compound have been given.

The results are summarized in Tables 1 and

                  TABLE 1                                                         ______________________________________                                        Determination of the degree of substitution (D.S.) at RT                        300 K      CL content      Formide content                                  H.sub.2 O content                                                                      D.S. max  D.S. min  D.S. max                                                                              D.S. min                                 ______________________________________                                        25       0.39      0.23      1.07    0.63                                        9 0.49 0.41 0.94 0.79                                                         9 0.43 0.37 0.57 0.49                                                         2 1.03 0.89 0.43 0.37                                                      ______________________________________                                    

                  TABLE 2                                                         ______________________________________                                        Determination of the degree of substitution (D.S.) at 80° C.             353 K      CL content      Formide content                                  H.sub.2 O content                                                                      D.S. max  D.S. min  D.S. max                                                                              D.S. min                                 ______________________________________                                        25       0.49      0.27      0.74    0.4                                         9 0.53 0.26 1.02 0.49                                                         9 0.37 0.34 0.48 0.44                                                         2 0.89 0.66 0.38 0.28                                                      ______________________________________                                    

From the test results, the following is evident:

1. The caprolactone content increases exponentially with a decrease ofthe water.

2. The proportion of formate decreases slightly with a decreasing amountof water.

Discussion

The observations made under points 1. and 2. can be explained asfollows: the increase of the proportion of caprolactone withsimultaneous decrease of the proportion of water reflects the decreaseof the irreversible hydrolysis of caprolactone which is determined bythe amount of water.

Formic acid does not react with water in a side-reaction; thus, theamount of formic acid remains constant. Since caprolactone has a higherreactivity towards the hydroxyl groups of starch than formic acid, itreacts faster with the hydroxyl groups which are free for acylation, andit reduces the amount of hydroxyl groups available for the slowerreaction with formic acid.

Although in each of the three preceding examples, caprolactone has beenused as a reaction component in the esterification of the starch or thestarch derivative, it is of course also possible to use other suitableesterification or transesterification partners to prepare athermo-plastically processible starch polymer component or mixture.Thus, in principle, in addition to caprolactone or generally lactones,esters, esteramides, dimeric fatty acids, modified fatty acids, acidmethyl esters, esterpolyols, glycerol trioleate and/or glyceroldilinoleate have proven to be suitable reaction partners. Also suitableare, of course, if appropriate, the polymers prepared from thesemonomers or oligomers, such as, for example, polyesterpolyol,polycaprolactone, polyesters prepared from the above-mentioned polyolsand fatty acids, polyesteramides, etc.

In each case, it is essential that the starch or the starch derivativeis brought to melt using suitable softeners or plasticizers prior tocarrying out the esterification or transesterification reaction inquestion, and to remove the softener or plasticizer used, such as, forexample, water or formic acid, at least partially, if appropriate, fromthe reaction mixture, for example by volatilization, when a certaindegree of substitution of the basic molar units of the starch isreached.

This is not necessary if the softener or the plasticizer simultaneouslyacts as reaction partner which participates in the esterification ortransesterification, for example of the starch derivative, as in thecase of caprolactone in the appropriate incipient swelling of starchdiacetate.

Finally, it is possible both to prepare appropriate molded articles,films or other extrudates directly from the esterification ortransesterification products of starch or starch derivatives preparedaccording to the invention, and to mix these esterification ortransesterification products initially with other, for examplehydrophobic, biologically degradable polymers, such as, for example,polycaprolactone, to prepare appropriate molded articles, extrudates andthe like from these polymer mixtures. Here, it is preferred to preparethe last-mentioned polymer mixture in one process step without firstisolating the esterification or transesterification product and tointroduce it once more into a plastification unit, such as an extruder.

We claim:
 1. A method for manufacturing a thermoplastically processablepolymer composition comprising:(a) blending at least one of starch or astarch derivative with at least one hydrophobic material selected fromthe group consisting of esters, lactones, polyesters, esteramides,polyesteramides, dimeric fatty acids, modified fatty acids, acid methylesters, esterpolyols, polyesterpolyols, glycerol trioleate, glyceroldilinoleate, and mixtures thereof, and (b) mixing and heating the starchor starch derivative with the hydrophobic material in a manner so as toform a thermoplastic melt and in order for at least a portion of thestarch or starch derivative to react with at least a portion of thehydrophobic material so as to form at least one condensation reactionproduct of the starch or starch derivative and the hydrophobic materialand in order to thereby form the thermoplastically processablecomposition.
 2. A method as defined in claim 1, wherein the starchderivative is a starch acetate.
 3. A method as defined in claim 1,wherein the starch or starch derivative is reacted with formic acid andat least one lactone.
 4. A method as defined in claim 1, wherein thestarch or the starch derivative is processed so as to include from 0.02to 0.5 parts by weight of water.
 5. A method as defined in claim 1,wherein the thermoplastic melt has a temperature in a range of 80° to200° C.
 6. A method as defined in claim 1, wherein the starch or astarch derivative initially includes a water content of about 2 to 10%by weight and is initially brought to a melt together with formic acid,wherein the thermoplastic melt is reacted with caprolactone and whereinat least a portion of the water and at least a portion of any unreactedformic acid is stripped off by volatilization from the thermoplasticmelt.
 7. A method as defined in claim 1, wherein the starch or starchderivative comprises starch diacetate which is brought to a melttogether with caprolactone, and wherein the starch diacetate andcaprolactone are at least partially reacted using a suitabletransesterification catalyst.
 8. A method as defined in claim 1, whereinplastification work of from 0.05 to 0.4 kWh/kg is applied to thethermoplastic melt.
 9. A method as defined in claim 1, wherein at leaststep (b) is carried out in a continuously operated kneader, twin-screwkneader/extruder, Buss cokneader and gear pump having a downstreamstatic Sulzer mixer.
 10. A method as defined in claim 1, wherein atleast step (b) is carried out in a time period from 2 to 30 minutes. 11.A method as defined in claim 1, further including the step of combiningat least one hydrophobic biologically degradable polymer with thecondensation reaction product of the starch or starch derivative and thehydrophobic material, wherein the hydrophobic biologically degradablepolymer is selected from the group consisting of aliphatic polyesters,polycaprolactone, polylactides, polyhydroxybutyric acid, copolymers ofvaleric acid, polyesters prepared by fermentation, copolyesters havingaromatic and aliphatic blocks, polyesteramides, polyesterurethanes,polyvinyl alcohol, ethylenevinyl alcohol, and mixtures thereof.
 12. Amethod as defined in claim 1, further including the step of combining atleast one natural polymer with the condensation reaction product of thestarch or starch derivative and the hydrophobic material, wherein thenatural polymer is at least one of gelatin, lignin, cellulose, aderivative of at least one of the foregoing materials, or a mixturethereof.
 13. A thermoplastically processable polymer compositioncomprising at least one condensation reaction product of starch or astarch derivative and a hydrophobic material selected from the groupconsisting of esters, lactones, polyesters, esteramides,polyesteramides, dimeric fatty acids, modified fatty acids, acid methylesters, esterpolyols, polyesterpolyols, glycerol trioleate, glyceroldilinolate, and mixtures thereof.
 14. A thermoplastically processablepolymer composition as defined in claim 13, wherein the compositionfurther includes at least one hydrophobic biologically degradablepolymer combined with the at least one condensation reaction product ofthe starch or starch derivative and the hydrophobic material, whereinthe hydrophobic biologically degradable polymer is at least one of analiphatic polyester, a copolyester having aliphatic and aromatic blocks,a polyesteramide, a polyesterurethane, polyvinyl alcohol, ethylenevinylalcohol, or a mixture of at least two of the above-mentioned polymers.15. A thermoplastically processable polymer composition as defined inclaim 13, wherein the composition further includes at least one naturalpolymer combined with the at least one condensation reaction product ofthe starch or starch derivative and the hydrophobic material, whereinthe natural polymer is at least one of gelatin, lignin, cellulose, aderivative of at least one of the foregoing materials, or a mixturethereof.
 16. A thermoplastically processable polymer composition asdefined in claim 13, wherein the polymer composition further includes atleast one of fillers, fibers or reinforcing materials.
 17. Athermoplastically processable polymer composition as defined in claim13, wherein the polymer composition further includes at least one ofadditives, softeners, pigments, or crosslinkers.
 18. A thermoplasticallyprocessable polymer composition as defined in claim 13, wherein thecondensation reaction product comprises up to 0.6 parts by weight ofstarch.
 19. A thermoplastically processable polymer composition asdefined in claim 13, wherein the polymer composition is at least one ofa film, tube, or extrudate.
 20. A thermoplastically processable polymercomposition as defined in claim 13, wherein the polymer composition isat least one of a molded article or an injection-molded material.
 21. Amethod as defined in claim 1, wherein the starch or starch derivativehas a degree of substitution of basic molar units of starch of up toabout 0.8 upon formation of the condensation reaction product.
 22. Amethod as defined in claim 1, further including the steps ofincorporating at least one of an additive, a softener, a pigment, or acrosslinker within the thermoplastic processable polymer composition andforming the composition into at least one of a film, tube, extrudate,molded article, or injection-molded material.